1. Field of the Invention
The present invention concerns an agile microwave bandpass filter having two-mode cavities that are parallel coupled, i.e. coupled through iris and screws, as opposed to line-structured filters.
2. Description of the Prior Art
There are two types of prior art agile microwave filters using parallel coupled cylindrical cavities, i.e. cylindrical cavities with parallel axes coupled together via an iris:
Single-mode cavity filters having the drawbacks of being relatively bulky, relatively costly and relatively long time to set up, because of the plurality of cavities.
Two-mode cavity agile filters in which two coupled orthogonal modes are generated in each cavity and which represent an improvement over the first type from the cost and setting up time points of view.
The accompanying FIGS. 1 and 2 are respectively views in perspective and diagrammatic transverse cross-section of a prior art four-pole agile filter which has two two-mode cavities of the TE11n type.
This filter thus comprises two cylindrical resonant cavities 1, 2 which are identical and which have parallel axes.
The internal volume of these cavities is adjustable by means of mobile pistons 3, 4 which enable adjustment of the resonant frequency of each of the cavities so that the tuning frequency of the filter can be shifted, i.e. to provide the required frequency agility of the filter.
As can be seen more clearly in FIG. 2, the filter input cavity 1 is fed by a rectangular waveguide 5 coupled to the cavity 1 by a coupling iris 6.
The cavity 1 thus resonates in the TE11n mode, for example, and provides the orientation of the electric field vector E1.
A 45.degree. coupling screw 7 enables another mode to be generated in the cavity 1, through disturbance of this cavity, at the same frequency but with an electric field vector E2 orthogonal to the first. A tuning screw 8 is used to adjust precisely the volume of the cavity 1 in order to compensate for the small disturbance in particular due to the coupling screw 7 and of the input iris 6.
Finally, the cavity 1 resonates in two TE11n modes, whence the name two-mode cavity.
The cavity 1 is coupled to the other cavity 2 by a coupling iris 19. By coupling of the magnetic field the latter allows the cavity 2 to resonate at the same frequency as the cavity 1, in a mode T11n with electric field E3.
As previously, a 45.degree. coupling screw 9 generates in the cavity 2 a second mode with electric field E4 orthogonal to the electric field E3. The mode E4 is magnetically coupled to the output waveguide 10 by a coupling iris 11. A tuning screw 18 is provided as previously.
The end result is a four-pole agile bandpass filter using only two "parallel" coupled cavities 1, 2, i.e. with a coupling parallel to the axis of cavities 1 and 2.
Of course, a six-pole agile bandpass filter may be obtained in the same manner by using three two-mode cylindrical cavities with parallel coupling, or more generally, an agile bandpass filter with 2 n poles (where n is an integer) may be obtained using n parallel two-mode cavities each coupled to the next by an iris, in the manner that the cavities 1 and 2 are coupled as shown in the drawing.
The above two-mode cavity agile filters have the following advantages:
they are simple to control, because of the double resonance per cavity, which greatly simplifies the mechanical construction of the filter; PA1 the total travel of the adjuster pistons 3, 4 is relatively long (around 10 mm), and enables a great accuracy in the frequency tuning with frequency agility.
However, they have the major disadvantage of an asymmetric frequency response resulting of the disturbances brought by the input and output irises, which is not the case with single-mode cavity filters.
The invention is directed to remedying these disadvantages.